My friend and I were talking about engines and he brought up and interesting point that I had never really thought about. When starting any internal combustion engine whether it be a single cylinder 2 stroke or a 6.0 what controls the direction of airflow through the engine. In other words what determines whether air enters from the exhaust side of the engine versus the intake side of the engine? I know this sounds really stupid but it's bothering both my friend and I.

Timing and pressure are what determine flow direction for internal combustion engines.

Given a two-valve head atop one cylinder with a single piston on a four-stroke engine for simplicity:
On the intake stroke the one valve opens while the other valve is closed while the piston is descending. Negative pressure induces flow into the combustion chamber. Intake. This only makes sense if the valve opening also introduces fuel to be drawn into the combustion chamber.

For the exhaust stroke one valve opens while the other valve is closed while the piston is rising. Positive pressure induces flow out of the combustion chamber. Exhaust. Whatever was in the combustion chamber is forced through the open valve. This makes sense for exhaust.

Of course there are two other strokes: compression and expansion where both valves are closed for the 4-stroke cycle.

The 2-stroke works the same way just with two fewer strokes. Simplistically, the intake and exhaust are opened on portions of both upward and downward travel of the piston; the direction of the piston up or down dictates flow direction: piston up = exhaust, piston down = intake...the exhaust valve opening is typically of longer duration.

This all gets more complicated with shock wave propagation and cam shapes for specific times, magnitude, and durations of valve opening. The air has mass and inertia and reflects (bounces) within the combustion chamber and cylinder.

All above is correct, but you can make a 2 stroke and a diesel run backwards. This is how you get reverse on sleds, it actually shuts the engine down and changes the direction of the crank making it run backward. I have also seen a diesel run backward, it was a very old front loader, while loading gravel it stalled and started to roll back, causing the engine to restart but backwards. It was pulling air from the exhaust and blowing smoke out of the air filter.

I hate this analogy, but I will use it here, an engine is a air pump so it will pump air both ways it just depends what way the crank is spinning.

I don't think a modern diesel engine could actually run backward with all the sensors on them now and without catastrophic failure.

I was under the interpretation that on a 6.0 the injector shot fuel into the combustion chamber itself not the intake valve? Where are the injectors positioned on a 6.0? Where are the spraying fuel into exactly?

I was under the interpretation that on a 6.0 the injector shot fuel into the combustion chamber itself not the intake valve? Where are the injectors positioned on a 6.0? Where are the spraying fuel into exactly?

The video above is of a gas engine... that's why it has a spark plug.
A DI diesel engine would have the injector about where the spark plug is on a gas engine

It doesnt matter if its an air fuel mixture or a direct injection, so it doesnt matter where fuel is sprayed. What matters is which valve is open on which stroke of the engine. In larmen's terms, the intake lobe on the cam makes the intake valve open when the piston is in the intake stoke pulling air from the intake or in our case getting forced in (turbo). The exhaust lobe on the cam opens the exhaust valve in the exhaust stroke and pushes the exhaust gases out of through the exhaust. The compression and powerstroke both, or all four valves are close.

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